Transmission system for sound carrying currents



1933. o. a. HARRIS ET AL 1,919,719 /5 TRANSIISSION SYSTE FOR SOUNDCARRYING CURRENTS Filed June 13, 1930 3 Sheets-Sheet 1 July 25, 1933.HARR|5 g- AL 1,919,719

TRANSMISSION SYSTEM FOR SOUND CARRYING CURRENTS filed June 13, 1930 3Sheets-Sheet 2 lllfl llll Lwenlofir diam/413W July 25, 1933. D. B.HARRIS ET AL 'IRANSIISSION SYSTEH FOR SOUND CARRYING CURRENTS Filed June13, 1930 3 Sheets-Shqet 3 g-PAL noun'uu n Patented July 25, 1933 UNITEDSTATES PATENT OFFICE DONALD B. HARRIS AND ORVILLE W. KNAUSS, OF ST.PAUL, MINNESOTA, ASSIGNORS, BY DIRECT AND MESNE ASSIGNMENTS, TO PROGRAMSERVICE COMPANY, OF ST. PAUL, MINNESOTA, A CORPORATION OF DELAWARETRANSMISSION SYSTEM FOR SOUND CARRYING CURRENTS Application filed June13,

This invention relates to systems for transmitting sound-carrying andm1agecarrying currents from a central station to a number of distantlylocated stations through the medium of conductors and is an improvementupon the invention disclosed in our U. S. Letters Patent No. 1,733,194,issued October 29th, 1929. In said patent an operative system wasdisclosed especially adapted for program service work where programs,such as music, addresses and other important advantages of the originalsys-- tem was to eliminate interference caused by capacitative inductionof energy into other conductors neighboring or in the same cable withthe conductors utilized in our system, which capacitative induction isoften referred to as cross-talk.

\Vhile the said original system was found to be operative afterextensive use and to be reasonably successful, we have found certainobjections thereto which the present invention overcomes.

Even though an efiicient transformer system was utilized in connectionwith the circuits, we found that cross talk or capacitative inductionfrom neighboring conductors or other wires in the same cable toconductors utilized in our system was not entirely overcome due to the'fact that the line conductors of our systemwere not balanced withrespect to ground. This unbalance to ground furthermore caused crosstalk or capacitative induction from our said line conductors toneighboring conductors used for telephone and other communicationpurposes.

The original system included provision for volume control at the distantstation, but this volume control was not entirely satisfactory 1930.Serial No. 460,882.

since it depended for its operation upon a regulation of filamentcurrent and consequently of filament emission, resulting in anundesirable change in the plate impedance which of course caused somedistortion.

Furthermore, the old system made no rovision whereby the filamentcircuits 0 remotel located amplifiers could be disconnccte by theoperators at the central office.

It has been found that the subscribers, where 60 they are paying forprogram service, often keep the filament circuits of their amplifiersconstantly connected. Itis found desirable to remotely disconnect, fromthe central ofiice, the said filament circuits of the distant amplifiersfor effecting economy in amplifier tube replacement and for economizingupon the use of amplifier supply currents.

Another objection to the old system was that a small amount of low fruency re eneration resulted in the system no to a eed back from the)late circuit to the grid circuit and throug 1 the medium of a otentialenerated along the plate circuit si e of the It is an object of ourpresent invention to provide an economical and highly efiicient systemfor use in program service work for transmitting sound-carryingcurrents, such so as audio-frequency and carrier-frequenc currents orfor image-carryin currents, suc as are used in television, sai currentsbeing transmitted from a central to a multiplici of distant stationsthrough conductors, su as, for example, the conductors or airs in aconventional communication ca 1e and wherein provision is made forovercoming the objections to said original system which are hereinbeforereferred to. w It is'a further object to provide a system of the classdescribed; wherein conductors, such as, for example, pairs in atelephone cable, may be utilized for transmitting sound andimage-carrying currents; wherein the same conductors may also beutilized in conjunction with ground for transmitting electromotiveforces for maintaining the elements of the distant amplifiers at oerative poten- I00 tials; wherem capacitative 1n uctlon or cross talkbetween conductors in our system and nei hboring conductors or otherconductors in t e same cable will be entirely eliminated; or morespecifically wherein the line conductors extending between the centralstation and the distant stations will be perfectly balanced in respectto ground at all times for the purpose of preventing furtherinterference.

Another object is to provide a system of the class described wherein thesignal currents will be amplified at the distant station to valueswhereby loud speakers or other reproducers may be operated to capacity;wherein the amplitude of signals delivered to the reproducers may beregulated without producing distortion; wherein the connection ordisconnection of the filament circuits of the amplifiers at the distantstations may be controlled from the central ofiice, and wherein meansare provided for preventing a transfer of energy from the plate circuitto the rid circuit thereby removing any chance for ow frequencygeneration.

These and other objects of the invention will be apparent from thefollowing description made in connection with the accompanying drawingswherein three forms of the invention are schematically shown and where-1n:--

Fig. 1 is a diagram of my preferred system, and

Figs. 2 and 3 are diagrams of modified systems.

Referring to the preferred form of the invention illustrated in Fig. 1,the elements and apparatus located at the left of the heavy broken lineA-A are located at the central station while the apparatus and elementsat the right of said line are located at one of the distant stations,such as the home of a subscriber to the program service.

At the central station a source of sound or image-carrying current isavailable from the output of a suitable amplifier and in the drawingsthe plate of the last audion tube is indicated by the numeral 4.Batteries 8 and 9 or other sources of direct current supply are alsolocated at the central station and a step-down transformer is located atthe central station, the secondary windings 6 and 7 thereof beingseparated by a condenser 40 and connected with a pair of conductors Xand Y, which, if desired, may be a pair of wires in a conventionalcommunication cable, said conductors extending from the central to thedistant station. The primary 5 of the step-down transformer at thecentral station is directly connected to the plate 4 and the secondarycomprising sections 6 and 7 is divided as indicated in the drawings. Band C potentials are applied to the conductors X and Y of the linethrough windings 6 and 7 from batteries 8 and 9 respectively. A key 10is located at the central ofiice for the purpose of operating a remotelylocated relay at the receiving amplifiers at the distant stations, thecontacts of said key opening and closing the filament circuit of thedistant amlifier. An amplifier and loud speaker are ocated at thedistant station. The push-pull circuit is preferably employed in theamplifier. This circuit affords a greater power output for the sameplate and grid potentials. The push-pull circuit moreover reducesdistortion to some extent by eliminating certain harmonics of thefundamental frequencies generated in the tubes.

In particular the structure of the amplifier at the distant stationembodies, choke coils 11 and 12 and condensers 30 and 31 for the purposeof segregating the supply circuit rom the program circuit; an inputtransformer comprising windings 18, 19, 25 and 26, the primary windingsof which are separated by a condenser 29; a potentiometer comprisingwindings 34 and 35 and having two movable arms 36 and 37; two thermionicvacuum tubes connected in push-pull of which elements 27 and 28 are thegrids of the tubes, elements 16 and 17 are the filaments and elements 23and 24 are the plates; an out at transformer with the windings 21, 22 an32, of which windings 21 and 22 are the primary, and winding 32 is thesecondary; a dynamic speaker including a voice coil 33 and a field coil20, which is also employed as a choke in the plate circuit for thepurpose of further segregating the supply and program circuits; a relayincluding a spring assembly 14 and a winding 14a; a power transformer 13and a key 15; a. resistance balance 50.

In describing our system, the apparatus and elements utilized will bedescribed in conjunction with the several circuits between the centralstation and the subscribers home.

Filament circuit The filament circuit for the thermionic tubes of theamplifying set at the distant station ma be conveniently supplied fromthe househo d source of alternating current. The filament circuitincludes the secondary winding of transformer 13, the contacts of relay14 and the filaments 16 and 17 of the two tubes and key 15 connectedwith the other side of the secondary winding of transformer 13. Thiscircuit is closed and suplies current to the filaments of the tubes asongas both the contacts of relay 14 and the contacts of key 15 areclosed. Key 15 is available to the use of the subscriber enabling thesubscriber to turn the amplifier on or off at will, while relay 14 isenergized by currents transmitted from the central station, therebymaking it possible for the operators at the central station to energizeor de-en- .ergize the filament circuits of the distantly The 0 erationof relay 14 will be further treated in the explanation of the controlcircuit.

Grid supply circuit means of condenser 29.

Signal circuit erated by the amplifier 4 and are impressed upon primarywinding 5 of the central ofiice output step-down transformer causingcurrent to flow through winding 5 and inducing a similar potentialacross windings 6 and 7. This potential causes alternating current toflow in the line from a oint P on the drawings through conductortransformer winding 19, condenser 29, winding 18 and back to the centralstation through conductor Y to the point Q. This current induces apotential into windings 25 and 26, said potential impressing analternating voltage on points M and N. This alternating voltage causescurrent to flow from M to N through potentiometer windings 34 and 35,setting up a corresponding alternating voltage between the potentiometertaps 36 and 37 and accordingly producing an alternating voltage be tweengrids 27 and 28 of the two tubes. The alternating polarity of the grid27 is always opposite to that of grid 28. Due to the normal operation ofa vacuum tube used as an amplifier, a corresponding alternating volta eof greater amplitude is generated in the plate circuits of the two tubesimpressing an alternating voltage between plate 24 and plate 23. thealternating potentials of said plates being of course always opposite.Due to said potential, a further current is caused to flow throughwindings 21 and 22 of the output transformer of the amplifier. Thesecondary coil 32 of the output transformer is connected with the voicecoil winding 33 of the dynamic speaker and an induced current andvoltage is generated in said secondary and delivered to the loudspeaker. At the distant station it will be noticed that a choke coil 20is provided, one of the terminals thereof being connected with a centraltap on windings 21 and 22 of the output transformer and the otherterminal of which is connected with one terminal of condenser 31 which,with condenser 30. is interposed between the choke coils 11 and 12. Thefunction of the choke 20 is to prevent passage of any signal currentswhich may be generated in the supply circuit due to an unbalance inwindings 21 and 22 of the primary of the output transformer or due to adifference in the amplifying properties of the tubes. If such currentswere allowed to traverse the supply circuit they would flow from thecentral tap of windings 21 and 22 through choke 20 and through condenser31 to the filament and due to the finite impedance of condenser 31,would set up a potential across this condenser which in turn would causea current through windings 18 and 19 and which would induce a potentialon grids 27 and 28 of the tubes causing the effect known as lowfrequency regeneration or motor boating.

The further purpose of choke 20 may be to supply or energize the fieldcoil of the dynamic speaker. In such a case the field is employed as thechoke for the above described purpose.

The combination comprising choke coils 11 and 12 and condensers 30 and31 embodies a conventional electrical filter. This filter is placedacross the line conductors primarily for the purpose of segregating thesignal currents on the conductors from the currents generated in theplate circuit of the tube. Although in a circuit of the push-pull type,signal currents in the supply circuit of the amplifier are small and arelimited to those generated due to unbalances in the tubes or in theoutput transformer, these currents are of sufficient magnitude to causeundesirable regeneration effects if they are allowed to return to thegrid circuit of the amplifier. In this invention signal currents ofsmall magnitude may be created in the circuit from the center tap ofwindings 21 and 2:2 through choke coil 20 to condenser 31. They are,however, prevented from returning to line conductor Y by the combinationof condenser 31 and choke coil 12. Condenser 31 offers a very lowimpedance ath for the return 01' these currents to the lament, whilechoke coil 12 offers a path of extremely high impedance. As a resultpractically no signal current is returned to the line conductors and theinput transformer, and regeneration is avoided. At the same time a pathof relatively low impedance is supplied by choke coil 12 for thetransmission of direct currens to the plate circuits of the tubes, whilecondenser 31, has, of course, an infinite impedance for currents of thistype and accordingly does not provide a path to the filament of thetubes.

Choke coil 11 and condenser 30 are inserted primarily for the purpose ofbalancing the line. conductors in respect to ground. The impedance ofthis combination is exactly equal to the impedance of condenser 31 andchoke coil 12 so that impedance from conductor X to ground is alwaysmaintained equal BOI to the impedance from conductor Y to ground.

A further object of the filter is to prevent the passage of signalcurrents on the conductors from conductor X to conductor Y. This objectis accomplished by the extremely high im )cdance of choke coils 11 and12.

ondenser 29 is inter osed between the primary windings 19 an 18 of thereceiving input transformer and has for its purpose to prevent a directcurrent short circuit across the line through said transformer windings18 and 19 while allowing the passage of signal currents through thetransformer for setting u alternating potentials for operation of thegrid circuit.

Plate supply circuit Both plates are supplied from potentialstransmitted over line conductor Y, to a circuit from positive B battery9 at the central office, key 10, winding 7, conductor Y, choke coil 12,choke coil 20, windings 21 and 22 and plates 23 and 24. As explained inthe description of the signal circuit, these plate supply potentialshave no effect on the input transformer of the amplifier due to the useof condenser 29.

Control relay circuit The control relay is energized by currentstransmitted from central ofiice battery 9 through the contactof he 10through winding 7, line conductor Y, c oke coil 12, winding 14a of relay14 to ground. By operating key 10 relay 14 may be energized orde-energized, thereby opening or closing the filament circuit.

Volume control operation As discussed briefly in the description of thesignal circuit the potential set up between points M and N causescurrent to flow through potentiometer windings 34 and 35 and sets up acorresponding otential between the potentiometer taps. he magnitude ofthe potential between the taps is directly proportional to theresistance included between them. The potentiometer is so constructedthat as the arms of the potentiometer are moved a similar resistancechange is produced between each tap and the center tapof thetransformer. A similar change in potential is correspondingly producedbetween each grid and the filament of the same tube. It wiil be notedthat the manipulation of the volume control has no efi'ect on the supplyconstants of the tubes.

Referring to the modified form of the invention illustrated in Fig. 2.the apparatus included in this form is similar to that described inconnection with the preferred form and the individual elements aredisposed similarly in respect to each other with the exception that aninput transformer with a sin le primary windmg is employed and that te-otentiometer used for volume control is p aced on the line side of theinput transformer and consists of a single winding. The primary of inputtransformer 15 numbered 18 and the two secondary windings are numbered25 and 26 in the drawing. The potentiometer winding is numbered 34' inthe drawing. With these exceptions the elements of the circuit aredesignated by the same numbers in the drawing as were employed in thecase of the preferred form.

In describin this modified form onl such circuits will be treated indetail as deviate from the corresponding circuits outlined in thedescription of the preferred form.

Filament circuit This circuit is similar to the preferred form.

Grid supply circuit This circuit is exactly similar to the grid supplycircuit described for the preferred form with the exception that gridpotentials do not traverse the potentiometer windings in conjunctionwith the secondary windings 25 and 26 of the input transformer.

Signal circuit In this circuit alternating signal potentials aretransmitted from the central station to the remote station in a mannerexactly similar to that employed in the case of the preferred form. Atthe remote station the signal potentials received from the line cause asignal current to flow through condenser 29' and potentiometer winding34' settin up a potential between points M and N indicated on thedrawing. This potential causes current to flow through winding 18' ofthe input transformer inducing a secondary otential across windings 25and 26 of the input transformer. This potential is applied directly tothe grids 27 and 28 of the amplifying tubes. The remainder of theoperation of the signal circuit is similar to the operation of thepreferred form.

Plate supply circuit This circuit is similar to the preferred form.

Oontrol relay circuit This circuit is similar to the preferred form.

Volume control operation As explained in the description of the signalcircuit, signal currents are caused to traverse the winding 34' of thepotentiometer. These currents cause potentials to be set up between anygiven mm on the length of the potentiometer winding and any other ointon the winding. These potentials are in all cases exactly proportionalto the resistance included between the two points. In this case apermanent connection is made to one end of the potentiometer. As themovable arm of the potentiometer is moved along the length of thewinding, the resistance between its point of contact and the end of thewinding is changed causing a corresponding change in the potentialapplied to winding 18' and resulting in a similar change in the signalcurrent flowing through winding 18'. It will be apparent that as thevolume control is manipulated no change is produced in the grid,filament or plate potentials of the tube.

Referring to the modified form of the invention disclosed in Fig. 3,this form of the invention is similar to the preferred form except thatthe filter combination 11, 30, 31 and 12 is omitted and is replaced bythe potentiometer combination 11", 29" and 12". This potentiometerperforms a dual purpose in regulating the volume and in maintaining the.line balance. The type of loud speaker shown is of the magnetic type,as com ared with the dynamic speaker shown in t e preferred form. Theconnections on the primary of the in ut transformer are also somewhatdifferent y made. This deviation will be discussed in explaining theoperation of the various circuits.

In describing this modified form, a complete ex lanation will be givenonly in the cases 0 circuits where marked deviations from the preferredform occur.

Filament circuit This circuit is similar to the preferred form.

Grid supply circuit Grid circuit potentials are supplied from thenegative terminal of C battery 8 through winding 6 of the central oiiiceoutput transformer, conductor X, potentiometer winding 11", transformerwinding 19, transformer windings 25 and 26 to the grids 27 and 28.

Signal circuit Alternating signal potentials are impressed betweenconductors X and Y at points P and Q as in the preferred form of theinvention. These potentials cause current to flow from point P throughline conductor X, potentiometer winding 11", condenser 29. potentiometerwinding 12", lme conductor Y to oint Q. This current sets up acorresponding potential between the potentiometer taps M and N which inturn causes current to flow through winding 19, condenser 30", condenser31" and winding 18. By induction a correspondin potential is createdacross windings 25 and impressed on the grid. These potentials are 26and is amplified and delivered to the loud speaker windin as describedin the explanation of the preferred form. Small unbalanced currentsgenerated in the plate supply circuit are prevented from returnin to thegrid circuit by choke coil 20", whic in this case does not form a partof the dynamic speaker, by condenser 31" which presents a low impedancefor the return of signal frequencies to the filament and by transformerwinding 18, which presents a high impedance to si nal currents andperforms the function of a retardation coil in addition to its purposeas a transformer.

The combination 19, 30", 31" and 18 performs the same function as thefilter 11, 30, 31 and 12 disclosed in the drawing of the referred form.By means of this filter a ow impedance path is provided for directpotentials transmitted to the grids and plates of the tubes, but a highimpedance path is presented between the line and the plate circuit topotentials originating either in the line or in the plate circuit. Bythis means attenuation of the line signal currents is prevented andcurrents generated in the plate circuits of the tubes are not allowed toreturn to the input circuit thereby preventing undesirable regenerationeifects. Windings 18 and 19 and condensers 30" and 31" are also balancedin such a way that the impedance from conductor X to ground is equal tothe impedance from conductor Y to ground thereby maintaining the lineconductors in erfect balance and further preventing or esseningundesirable interference efi'ects.

Plate supply circuit Control relay circuit Control relay is energized bycurrents transmitted from central otfice B battery 9 through thecontacts of key 10, winding 7, line conductor Y, potentiometer winding12", winding 18, relay coil 14a to ground. By operating key 10, relay 14may be energized or de-energized opening and c osing the filamentcircuit of t fier.

From the foregoing description it will be seen that the threeembodiments of the invention disclosed successfully fulfill the objectsset forth. Of the three forms disclosed, the form shown in Fig. 1presents certain advantages in relation to the modified forms, but eachof the modified forms possesses e remotely located amplilesser points ofadvanta e in certain respects.

Referring to the pre erred form, the employment of the volume control inthe grid circuit of the tubes is advantageous in that no current flowsthrough the volume control, effect-ing substantial economy in themanufacture of the windings. In order to make this advantage availableit has been necessary, in Fig. 1, to employ the separate segregatingfilter 11, 30, 31 and 12, as the use 0 the potentiometer acrossthesecondary reduces the primary impedance of the input transformer to apoint where it would no longer be effective as a segregating unit,particularly in view of the fact that an extremely high ratio inputtransformer must be employed in this case. A further advantage of theemployment of said segregating filter is that no direct current isallowed to traverse the transformer windings 18 and 19 therebyprfventing saturation effects which might resu t.

The form of the invention disclosed in Fig. 2 has the same advantages asdisclosed above in reference to the preferred form and has theadditional advantage that the form of potentiometer employed is of asimpler design resulting in economy of manufacture. This advantage,however, is thou ht to be outweighed by the fact that the form disclosedin Fig. 2 does not present as perfect a line balance as the preferredform.

The form disclosed in Fig. 3 presents an advantage in that a separatefilter combina-- tion is not employed, the windings of the transformeralso erforming the function of the filter. In ordbr to keep thetransformer windings at as high an impedance as possible, thepotentiometer is p aced on the line side of the transformer. In thisrespect the preferred form is preferable to the form disclosed in Fig. 3in that no current traverses the potentiometer windings in the preferredform and consequently an economy in manufacture is effected.

Figs. 1 and 2 show the use of the field coil of the dynamic speaker as achoke coil for impeding the passage of unbalance currents from the platecircuit to the supply circuit. This is an economical feature and s ouldresult in a lower cost of manufacture.

In the three embodiments of the invention heretofore disclosed it is tobe understood that the winding 5 of the central ofiice outputtransformer is so related to windings 6 and 7 in respect to turns ratiothat potentials impressed on conductors X and Y will be sufficiently lowin our system, where the line is perfectly balanced with respect toground,

that interference through capacitative induction or cross talk betweenconductors X and Y and other neighboring conductors or conductors in thesame communication cable will be entirely eliminated, it beingrememoabered that our system is especially adapted for use in programservice supplied by telehone companies, and where y conductors in theconventional communication cables may be utilized for transmitting soundor image-carrying currents, as well as electromotive forces formaintaining the grid and plate elements of the amplifier tubes atoperative potentials. The use of a line potential of this small mait-ude is made possible'by f so terminating tie line at the centraloflice that the impedance of the termination is equal to the lineimpedance. Under these conditions the transmission of a maximum energylevel can be accom lished with a corresponding minimum vo tage level andtransmission of said alternating current from the central station to thedistant station will be accomplished with maximum efliciency.

It will, of course be understood that various changes may be made in theform, details and particular types of apparatus used and that while thepush-pull amplifier circuit is illustrated, any other type of amplifiercircuit ma be utilized with our system, all within t e scope of ourinvention.

In the appended claims the term alternating currents is of course meantto include all currents described in the foregoing specification assound-carrying, image-carrying or signal currents.

It is further to be understood that in this application and the claimsthereof, wherever the word ground is to be used, it shall include earthor a common conductor of any nature, such as the sheath of aconventional communication cable, or a third conductor in a cable.

What is claimed is:

1. In transmission systems for alternating current, a central station, asource of alternating current and a source of direct current at saidcentral station, a distantly located station, a thermionic amplifier atsaid distant station having a grid element, a filament element and aplate element, a pair of conductors connected to transmit alternatingcurrent from said source to said amplifier, a circuit including saidpair of conductors and ground for supplying electro-motive forces fromsaid source of direct current to said amplifier whereby said grid andplate are maintained at operative potentials in respect to said filamentand a combined transformer and impedance system having a unit at saidcentral station connected to said pair of conductors and to ground andhaving a unit at said central station interposed between said conductorsand said source of alternating! current and connected to ground andhaving a unit at said distant station interposed be tween saidconductors and said amplifier and connected to ground, said transformerand impedance system being adapted to supply a terminating impedance forsaid line conductors equal to the characteristic impedance of 10 saiddistant station having a grid element,

a filament element and a plate element, a pair of conductors connectedto transmit alternating current from said source to said amplifier, atransformer system including a transformer at one of said stat-ions,windings of which constitute an electrical filter and having a unit atsaid central station connected to said source of alternating current andto said conductors and having a unit at said distantly located stationconnected to said amplifier and to said conductors and a cooperatingimpedance system connected to said conductors and to ground, saidimpedance system having a unit at said central station and a unit atsaid distant station,

one of said impedance units including the transformer windings whichconstitute said electrical filter, said impedance system, in conjunctionwith ground and said conductors, supplying electro-motive forces to saidamplifier whereby said grid and plate elements will be maintained atoperative potentials with respect to said filament elementandmaintaining said conductors in a balanced condition with respect toground.

3. In a transmission system for alternatingcurrent, a central stationhaving available a source of alternating current, a source of directcurrent, a distantly located station, a thermionic amplifier at saiddistant station having grid. filament and plate elements, a pair ofconductors connected to said source of alternating current and to saidsource of direct current, at said central station and connected to saidamplifier atsaid distant station, said conductors transmittingalternating current from said source to said amplifier, means formaintaining said grid and plate at operative potentials in respect tosaid fiament, a filament circuit for maintaining said filament at atemperature adequate for proper operation, a controlling element at saiddistant station including an electro magnet and a pair of contactsinserted in said filament circuit, said contacts being actuated by saidelectro magnet, and a control circuit for actuating said control elementincluding said electro magnet-and including a circuit breaking elementat saidcentral station connected to one of said conductors and to saidsource of direct current.

4. In a transmission system for alternating current, a central stationhaving a source of alternating current, a source of direct current, adistantly located thermionic amplifier having a grid, filament andplate, a filament circuit for maintaining said filament at a temperatureadequate for proper operation, a controlling relay at said distantstation having a pair of contacts inserted in said filament circuit andincluding an electro magnet, a pair of conductors extending between saidcentral station and said distantly located amplifier, said pair ofconductors being connected to said source of alternating current, tosaid amplifier and to said source of direct current for transmittingelectro motive forces to said amplifier whereby said grid and plate willbe maintained at operative potentials in respect to said filament andone of said conductors being connected to said electro magnet and acircuit breaking element at said central station interposed between saidsource of direct current and one of said conductors for controllingcurrents transmitted to said electro magnet.

5. In transmission systems for alternating current, a central statlonhaving available a source of direct potential, a source of directcurrent and a source of alternating current, a distantly locatedstation, a thermionic amplifier at said distant station having plate,

rid and filament elements, a pair of con-, ductors extending betweensaid sources and said amplifier and connected thereto, a circuit forsaid alternating current including said conductors, grid and platesupply circuits also including said conductors and including said sourceof direct potential and said source of direct current for maintainingsaid grid and plate elements at operative potentials in respect to saidfilament element and means for maintainin said pair of conductors in abalanced con ition with respect to ground.

6. In transmission stems for alternating current, a central stationhaving available a source of direct potential, a source of directcurrent and a source of alternating current, a distantly locatedstation, a thermionic amplifier at said distant station having grid,plate and filament elements, a connection to ground at said centralstation and at said distant station, a pair of conductors extendinbetween said sources and said amplifier an connected thereto, a circuitfor said alternating current including said conductors, a plate supplycircuit also including at least one of said conductors, includin saiddirect current source and including sai connections to ound, aconductive path connected to sai grid element including at least one ofsaid conductors including said connections to ground and including saidsource of direct potential adapted to maintain said grid element at anoperative potential in respect to said filament element and means formaintaining said pair of conductors in a balanced condition with respectto ground.

including sid directcurrent source, a com ductive path connected to said"rid element and including at least one of said conductors and includingsaid source of operative potential adapted to maintain said grid elementat an operative potential in respect to said filament element and meansfor maintaining said pair of conductors in a balanced cond1- tion withrespect to ground.

DONAIZD B. HARRIS. QRVILLE w. KNAUSS.

